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Chapter
Ultrasound of the Kidneys:
Application of Doppler and
Elastography
MoawiaGameraddin
Abstract
Doppler ultrasound of the kidneys is essential in the assessment and diagnosis
of kidney diseases. There are several diseases involving the kidneys. Some are
functional, diffuse and systematic. Using Doppler imaging provides an assessment
of vascular changes which is easily evaluated. Doppler investigation is widely used
for assessment of the perfusion of renal arteries. The Doppler indexes; resistive
index, pulsatility index, peak systolic are utilized for evaluating the blood flow of
the renal arteries. Doppler analysis provides useful diagnostic data that can predict
early damage of the kidney tissue. In recent years, ultrasound elastography showed
advanced development. It is a new promising technique that is used for assessing
the renal tissue characterization. Elastography is an effective imaging for assessing
kidney diseases. In the future, clinicians can use elastography instead of biopsy. In
this chapter, we highlighted the applications of Doppler ultrasound and elastogra-
phy in evaluation of various kidney diseases.
Keywords: Doppler, renal elastography, kidney disease, renal artery
. Introduction
Doppler ultrasound is widely used in medical imaging. It is an application of diag-
nostic ultrasound utilized for assessing the blood flow speed and direction. These mea-
surements depend on the Doppler effect is used to measure changes in the frequency
of the echoes reflected from moving blood cells. In many cases, Doppler ultrasound
replaces X-ray angiography. The most important advantage of Doppler ultrasound over
other imaging methods that it provides a real-time assessment of blood flow.
The Doppler renal resistive index (RRI) is the most common Doppler parameter
that is used to assess a variety of renal diseases such as assessment of rejection of
transplanted kidney, detection of renal artery stenosis in hypertensive patients and
evaluation of chronic kidney disease (CKD).
Ultrasound elastography is an advanced imaging method which is sensitive
to tissue stiffness. In recent years, elastography has been further developed to
enable quantitative assessments of tissue stiffness. Elastography is capable to assess
changed elasticity of soft tissues resulting from specific pathological processes. It
can differentiate between malignant and benign renal masses which may replace the
need of biopsy. The combination of Doppler and elastography provide rich diag-
nostic data about the pathological processes with kidney tissue which is essential for
management and treatment.
Essentials of Abdominal Ultrasound
. Ultrasound examination technique
The kidneys are examined with ultrasound in longitudinal and transverse scans
planes using . and MHz transducers. The organ is examined in supine position
combined with the lateral decubitus. Then various planes are performed to demon-
strate the entire kidney. Preferably, longitudinal and transverse planes are taken to
determine the length and size of the kidney, as shown in Figure .
In the adult patient, a curved array transducer with of .–.MHz is used, while
high-frequency –MHz is used in the pediatric patients.
Artifacts of the lowest ribs and gastric gases may obscure the upper poles of
the kidneys. However, the whole kidney can be investigated during either normal
respiration or breath hold, since the kidney will follow the diaphragm movement
and change position accordingly [].
. The Doppler ultrasound: a general review
Doppler ultrasound has been extensively utilized in assessing reno-vascular
diseases since it is a safe, non-invasive, available and cheap. These measurements
depend on the Doppler effect is used to measure changes in the frequency of the echoes
reflected from moving blood cells. In many cases, Doppler ultrasound replaces X-ray
angiography. The most important advantage of Doppler ultrasound over other imaging
methods that it provides a real-time assessment of blood flow.
. Types of Doppler ultrasound imaging
All kinds of Doppler sonography are widely used in medical imaging. The
advantages of these types are high accuracy in measurements, non-invasive nature,
accessibility, and no harmful biological effects. Today, there are three types:
a. Color Doppler
b. Power Doppler
c. Pulse wave Doppler
The color Doppler (CD) converts Doppler shifts to an array of colors and form
a picture of blood vessels to display the speed and direction of blood flow through
Figure 1.
The length and width of the kidney.
Ultrasound of the Kidneys: Application of Doppler and Elastography
DOI: http://dx.doi.org/10.5772/intechopen.85196
the vessels. The Doppler shift is the difference between the incident frequency and
reflected frequency. Positive Doppler shift occurs when the reflector is moving away
from the probe, and a negative shift occurs when the reflector moving toward the
source of ultrasound. Thus, the Doppler shift is directly proportional to the velocity
of the blood flow.
FD =
Fv cos
___________________
C ()
where F: is the transmitted ultrasound frequency; V: is the reflector velocity;
C: is the speed of sound; Cos : is the cosine of the angle between the transmitted
beam and the reflector path.
.. Factors influencing color flow image
. Power: transmitted power into tissue*
. Gain: affect sensitivity to flow signals
. Frequency: affect sensitivity and resolution. High frequency provides better sensi-
tivity to low flow while lower frequency has better penetration and lesser aliasing.
. Pulse repetition frequency (PRF): called scale: low PRF concerns at low veloci-
ties and high PRF reduces aliasing.
. Area of investigation: larger area reduces frame rate. Thus, reducing the color
box of the flow area under examination will usually improve frame rate and
may allow a higher color scan line density with improved spatial resolution
. Focus: should be coincide to the region of interest [].
.. Practical guidelines of color Doppler flow imaging
. Choose the set-up key. This improve Doppler parameters for specific
investigations.
. Apply power within the study area and then adjust color gain. Ensure focus is
set at the level of region of investigation. Adjust gain to improve color signal.
. Position beam steering to get satisfactory beam angle for the selected artery or
vein.
. Adjust PRF to synchronize the flow status. Low PRF are very sensitive to low
flows or velocities but may cause aliasing. High PRF decrease aliasing but are
less sensitive to low flows/velocities [].
. Set the color flow area to suitable size. A small color flow ‘box’ or region may
lead to a better frame rate and better resolution.
.. Spectral wave Doppler
Pulsed wave Doppler (PWD) ultrasound is used to generate a sonogram of a
blood vessel (vein or artery) under study (Figure ). PWD provides a measure of
Essentials of Abdominal Ultrasound
the flow changing velocity throughout the cardiac cycle and display distribution of
velocities in the sample volume (gate) as demonstrate in Figure . Velocities can be
measured when an accurate angle correction is made.
.. Factors affecting the spectral Doppler image
. Power: set the transmitted power to study area.
. Gain: influence sensitivity to flow signals.
. PRF: low PRF is used to detect low velocities while high PRF decrease aliasing.
. Gate size: beam steering allows improved beam angle for accuracy of calcula-
tion of flow velocity.
.. Guidelines for practical spectral Doppler image
. Set power to the selected study area.
. Place the Doppler cursor on the artery/vein to be examined.
. Gain should be adjusted so that the image is clearly visible and noiseless.
. Apply the beam steering to get a satisfactory angle. Remember that angles
approaching to ° will give ambiguous image or unclear data. The beam angle
must be ° or less when velocity measurements are to be maintained.
. Adjust the PRF/scale and baseline to suit flow conditions. The sonogram
should be clear and not subjected to aliasing.
. Adjust the sample volume (SV) to correct and suitable size coincided with area
under investigation. Correct the angle to obtain accurate velocities. Use the
B-mode and color flow image of the vessel to make the angle correction [].
. Doppler ultrasound of the kidneys
Doppler ultrasound is essential for evaluation of the kidneys. Doppler is consid-
ered more accurate than conventional sonography since it provides functional and
Figure 2.
Spectral wave Doppler. Renal arterial velocity waveform.
Ultrasound of the Kidneys: Application of Doppler and Elastography
DOI: http://dx.doi.org/10.5772/intechopen.85196
vascular information which are lacked in grayscale ultrasound. Doppler ultrasound
assesses patterns of renal and extrarenal vascularization [].
Doppler investigations must be performed properly to gain useful data. It allows
information about the presence and direction of blood flow in renal vessels. Renal
artery stenosis can be assessed by Doppler indices; resistive index (RI), pulsatility
index (PI) and systolic to diastolic ratio (S/D). These indices provide hemodynamic
and predictive information regarding the renal arteries. Analysis of the RI may
provide helpful clinical information in various renal diseases [].
.. Doppler procedure of the renal arteries
The investigation starts with the patient in the supine position using a low-fre-
quency probe (.–.MHz) to depict the abdominal aorta (AA) and renal arteries
(RAs). The two main approaches for imaging the renal arteries are through the
anterior abdominal wall. In most situations the anterior approach is used to assess
the main RAs [, ].
The RAs arise from the lateral borders of the abdominal aorta (AA) at the level
of the second lumbar vertebra, almost –cm inferior to the superior mesenteric
artery (SMA) origin. The right RA arises from the anterolateral aspect of the
abdominal aorta and it courses under the inferior vena cava (IVC) [–]. From
this view, RA flow is in a direction that is parallel to the Doppler beam, optimiz-
ing signal reception. The patient usually needs to be placed in the opposite lateral
decubitus position [, ].
A .MHz curvilinear array transducer with variable focal zone are used. The
Doppler examination is usually performed in supine positions as stated by the renal
ultrasound protocols. Each Kidney will be examined firstly with B-mode ultrasound
in at least two planes to maintain the renal length for each kidney. The Doppler indices
(RI and PI) are measured at interlobular or arcuate artery in the upper, middle, and
lower portions of the kidney and the mean values were calculated for each kidney.
.. Normal vascularity of the renal artery
Doppler RI is efficient to detect intrarenal vascular pathological processes.
Several studies have demonstrated that a normal mean renal RI is approximately
.. It was reported that a mean RI of .±. for individuals without
Figure 3.
Renal artery spectral Doppler demonstrates renal artery stenosis; PSV is 452.4 cm/s, RI= 0.80 in a 25-years
male with hypertension and abnormal renal function (the sonogram taken by Dr. Moawia Gameraddin).
Essentials of Abdominal Ultrasound
pre-existing renal disease []. Other studies also reported normal mean RI values of
.±., .±. [], and .±. [, ]. In addition, most sonographers
have considered . to be the upper threshold of the normal RI in adults [, ].
.. The importance of Doppler resistive index
Doppler sonographic analysis of renal artery waveforms was empirically applied
to disease characterization (Figure ). Despite RI is a good predictor of several
renal abnormalities, there are factors that affect the arterial waveform such as
vascular resistance, vascular compliance, and heart rate. In a previous study, it was
reported that renal RI was associated with “histological changes and poor renal out-
come during chronic kidney disease”. It was shown that RI≥. is associated with
arteriosclerosis, severe interstitial fibrosis and renal function decline. Therefore, RI
is essential Doppler parameter that contribute to diagnose patients at high risk of
end-stage renal disease (ESRD) [].
.. Application of Doppler in renal diseases
... The role of Doppler in hypertension
Hypertension involves approximately – of the adult population and it was
reported that the prevalence will increase. It is considered a main risk factor for the
development of renal failure and cardiovascular disease. It was reported that
of patients with chronic kidney disease (CKD) are hypertensive. The relationship
between hypertension and kidney disease is complex and it is attributed to the inter-
related pathophysiology. Renal hypertension or renovascular hypertension means
hypertension due to renal artery stenosis and kidney disease. Thus, patients who
newly diagnosed hypertension must be screened for underlying kidney disease [].
... The role of Doppler RI in renal hypertension
The Doppler RI has been utilized for many years in a variety of clinical situ-
ations. Doppler ultrasonography detects renal abnormalities at macrovascular
and microvascular levels. Assessment of renal RI at different regions of the renal
parenchyma may suggest physiological or morphological changes within the
kidneys. Therefore, it provides useful information for diagnosis and prognosis of
the disease.
Recent studies revealed an increased renal resistive index (RRI) in patients
with primary hypertension not only reflects vascular changes in intrarenal sup-
ply, but that it is also associated with atherosclerosis and systemic hemodynamics.
Therefore, it provides useful prognostic information.
.. Doppler assessment of non-obstructive diseases
... Acute and chronic kidney disease
Acute kidney injury: Acute kidney injury (AKI) was reported to associate with a
high morbidity, long-term mortality and apparent economic impact [].
Doppler ultrasound has been widely used in the assessment of renal diseases
for diagnosis, prognosis and management. Doppler ultrasound is non-invasive,
low cost and safe method for the evaluation of the renal blood flow. Recent stud-
ies reported different incidence of AKD among hospitalized patients classified as
Ultrasound of the Kidneys: Application of Doppler and Elastography
DOI: http://dx.doi.org/10.5772/intechopen.85196
“KDIGO classification (. ), followed by AKIN (. ) and RIFLE (. )
and CK (. ).”
... Doppler evaluation of acute kidney injury
In gray scale ultrasound, AKD reveals increased renal parenchymal echogenicity
which attributed to inflammatory states (acute glomerulonephritis, acute interstitial
nephritis, acute tubular necrosis, HIV nephropathy) or infiltrative diseases (lym-
phoma, monoclonal, myeloma and gammopathies) decreased thickening of kidney
cortex and echogenicity are also significant findings of AKI []. Color Doppler iden-
tifies the renal vessels localization to calculate RRI to monitor renal perfusion. In late
stages of AKI, RRI usually exceed ., and a threshold of . is reported as optimal in
recognizing between renal and prerenal disease. However, RRI values lower than .
are related to a good recovery after fluid rehydration, while RRI >. suggest a devel-
oping ischemic acute tubular necrosis (ATN) and worse prognosis []. In conclusion,
RRI play an effective role in different types of AKI.
.. Doppler assessment of chronic kidney disease
Chronic kidney disease (CKD) is considered as one of the public health problems
worldwide []. According to the report of Global Burden of Disease in [],
CKD had been ranked the first cause of death worldwide at th to th over two
decades. It was reported that “the surge of the CKD epidemic over these decades
produced an increase in years of life lost related to CKD, a disease toll of the
same magnitude of that attributable to diabetes”.
Ultrasonography of the kidneys is essential imaging modality among other
renal imaging methods since it is available, low cost and safe. US can easily assess a
CKD by measuring the length of the kidneys and evaluating the echogenicity of the
kidney cortex. The reduction of size and increased echogenicity reflect pathological
processes within the kidney.
The normal kidney length is about –cm (the left kidney is about mm
longer than the right kidney) in younger adults and a progressive atrophy with
aging. Normal kidney is always as bright as normal liver or spleen tissue []. When
the kidney cortex became brighter (echogenic) than hepatic tissue or splenic tissue,
Figure 4.
Duplex Doppler reveals normal waveform of the renal artery (a sonogram taken from Awadia Gareeballah
and Moawia Gameraddin researches).
Essentials of Abdominal Ultrasound
this reflects inflammatory changes in the kidney tissues. CKD is often associated
with increased echogenicity of the renal cortex since fibrous tissue such as glomeru-
losclerosis interstitial fibrosis, increases echogenicity.
However, those inflammatory conditions such as glomerulonephritis and acute
interstitial nephritis (ATN) are associated with hyperechoic aspect of the renal
parenchyma. In most cases, small and echogenic kidneys always suggest CKD
instead of AKI.
Doppler ultrasound plays effective role in defining CKD and its progression
to ESRD.Renal RI is reported to be correlated with arteriolosclerosis, glomerulo-
sclerosis and tubulointerstitial lesions more than others morphologic parameters
like kidney length and cortex area []. In general, higher values of renal RI
(>.) ordinary reflects more severe arteriolosclerosis than normal values (<.)
or high normal RRI (.≤RI<.) []. However, patients with high-normal
renal RI revealed good response to steroid therapy compared to a RRI>. [].
Additionally, patients with advanced CKD stage showed significant higher RI than
patients with earlier CKD stage.
.. Renal masses
Ultrasound plays a key role in screening renal cancer in asymptomatic patients.
Most renal tumors remain are not accurately diagnosed on US and require CT for
further characterization. However, US help to characterize cystic RCC that remain
unclear on computerize tomography (CT). Recent technology in gray-scale imaging
have improved the accuracy of US in the diagnosis and staging of kidney cancer. In
addition, solid renal masses can grossly be categorized as completely solid, multifo-
cal, or partially cystic tumors. The cystic appearance is mainly due to necrosis.
... Ultrasound evaluation kidney cancer
Computed tomography (CT) is the gold standard for imaging the kidneys. It is
accurate for detecting and characterizing renal neoplasms and staging renal cell
carcinoma (RCC). On the other hand, ultrasound (US) has a less sensitivity in
detecting small renal lesions, but it plays a key role in the early diagnosis of kidney
cancer since it is routinely used in the evaluation of the abdomen. Renal masses
were identified on US as a distortion of the normal tissue echotexture. Previous
studies reported that RCC is detected incidentally in asymptomatic patients. Only
of patients with RCC present with the classic triad of hematuria, pain and a
flank mass. Most of these patients often have advanced disease. More than of
the present with none of these three symptoms []. The RCC might be detected
incidentally during abdominal sonography. The majority of RCC measure less than
cm or less on US.Early detection of RCC improves prognosis and survival rate.
The sonographic appearance of renal tumors vary between isoechoic-,
hypoechoic, and hyperechoic compared with the normal renal parenchyma [].
Doppler US assesses the blood flow patterns of vascularity in renal tumor tissue.
It reveals vessels with high velocities. In RCC, the hypervascularity is attributed
to neovascularization. The Doppler RI on spectral Doppler US was reported to be
useful in detecting RCC in patients with ESRD [].
.. The Doppler assessment of transplanted kidney
US is the most imaging modality for assessment of the transplanted kidneys
(TK) (Figure ). The TR is located in the right or left iliac fossa. The superficial
location of the graft make the US examination accurate and ideal. The renal graft
Ultrasound of the Kidneys: Application of Doppler and Elastography
DOI: http://dx.doi.org/10.5772/intechopen.85196
is vulnerable to several pathologic changes which might occur immediately or later.
The sonographic appearance for evaluation of immediate post-plant pathologies
may not be specific such as acute tubular necrosis (ATN), acute rejection, and toxic-
ity associated with immunosuppressive calcineurin inhibitors [].
The Doppler renal RI has a significant correlation with renal allograft size. It
was reported that RI of . or higher was considered a strong predictor of graft
failure and morphological changes []. The increase of RI was reported to correlate
with presence of acute rejection and ATN []. On the other hand, elevated serum
creatinine levels in renal transplant patient reveals high RI values. Therefore, the
renal RI was a good predictor of graft function.
. Renal elastography
Ultrasound elastography (USE) was first described in the s. It is an imag-
ing technology which is sensitive to tissue stiffness. In recent years, elastography
has been further developed to enable quantitative assessments of tissue stiffness.
Elastography is capable to assess changed elasticity of soft tissues resulting from
specific pathological or physiological processes []. For example, tissue of solid
tumors tends to differ mechanically from surrounding healthy tissues. Furthermore,
fibrosis makes diseased tissue to be stiffer than normal ones. The role of elastography
is to differentiate diseased tissue from normal one for diagnostic applications.
Ultrasound elastography (USE) of the kidneys is a potential application is an
advanced imaging tool that may become a clinical biomarker for disease. However,
elastography of renal transplant cortex and the corticomedullary strain ratio have
been studied and they were found to correlate with renal cortical fibrosis [, ].
Shear-wave elastography (SWE) of the kidney utilizing acoustic radiation force
impulse (ARFI) is a potential clinical application which was reported to demon-
strate successful clinical applications in human organs []. In the kidney, SWE has
shown promise in the evaluation of CKD, renal transplant function, and renal vein
thrombosis (RVT).
. Renal fibrosis
USE is clinically useful to detect and assess fibrosis in CKD and transplanted
kidneys. USE with both strain imaging and SWI methods are noninvasively to
Figure 5.
A sonogram of a transplanted kidney shows normal size and normal color flow.
Essentials of Abdominal Ultrasound
detect, stage and monitor kidney fibrosis, thus, reducing the need for renal biopsy
[]. SWI is preferable to strain imaging in evaluating kidney fibrosis in both renal
graft and native kidneys since it is independent of external compression []. A
previous study reported that SWE renal stiffness was higher in patients affected
with CKD than in healthy controls []. Therefore, tissue stiffness measured by
USE was significantly correlated with histopathologic renal fibrosis. This finding
concluded that, USE is a non-invasive tool for predicting kidney fibrosis.
. Characterization of focal renal lesions using elastography
USE is useful for characterizing focal renal masses since US features are
not specific for malignancy. Assessment of renal masses with USE have shown
controversial results. Some results found SW velocity values could differenti-
ate between benign and malignant masses. Another study compared between
malignant and benign renal masses concluded that malignant tumors are .
times stiffer than benign masses []. A previous study reported that USE can
differentiate between renal cell carcinoma (RCC) and transitional cell carcinoma
(TCC) []. In general, quantification of kidney tissue using USE is more com-
plex than other organs since the high heterogeneity of the renal tissue. However,
the combination of doppler ultrasound and elastography will provide better
assessment of kidney abnormalities as compared in Table .
. Conclusion
In summary, Doppler ultrasound and USE are very effective imaging method to
the kidneys. Doppler assesses vascularity of the kidneys while elastography evalu-
ates tissue elasticity. USE is a new developing method and various studies have been
made using elastography in kidneys. It is very effective on the transplanted and
CKD kidneys to evaluate the corticomedullary fibrosis to prevent invasive biopsy.
Criteria Doppler ultrasound Elastography
Main principle Evaluates vascularity Assesses elasticity
Renal transplantation Renal blood resistivity index (renal RI) above
. indicates renal graft dysfunction.
Assesses cortical fibrosis in
early stage. Additionally,
elastography assesses the
grades of fibrosis; distinguish
mild from moderate fibrosis.
Obstructive and
non-obstructive
hydronephrosis
Doppler US distinguishes between
obstructive and non-obstructive
hydronephrosis. Obstructive hydronephrosis
reveals higher RI values than non-obstructive
hydronephrosis.
Measurements did not enable
distinguishing of obstructive
hydronephrosis from non-
obstructive hydronephrosis in
children
Differentiation
between malignant
and benign tumors
Doppler US is useful in characterization
of renal pseudotumors. Doppler allows
differentiation of normal vascularity from
tumor neovascularity. On the other hand,
benign renal masses characterized by less
or peripheral vascularity, homogeneous
echotextures and well-defined margins.
Malignant tumors are stiffer
than benign masses.
Table 1.
Comparison between Doppler ultrasound and elastography in evaluation of abnormalities of the kidney.
© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/.), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Ultrasound of the Kidneys: Application of Doppler and Elastography
DOI: http://dx.doi.org/10.5772/intechopen.85196
Conflict of interest
The author declares there was no conflict of interest regarding this chapter.
Author details
MoawiaGameraddin
Department of Diagnostic Radiologic Technology, Faculty of Applied Medical
Sciences, Taibah University, Almadinah, Kingdom of Saudi Arabia
*Address all correspondence to: gameraldinm@gmail.com
Essentials of Abdominal Ultrasound
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DOI: ./s---
